Direct numerical simulation analysis of spanwise oscillating Lorentz force in turbulent channel flow at low Reynolds number

Atilla Altıntaş,Lars Davidson

doi:10.1007/s00707-016-1754-7

Atilla Altıntaş, Lars Davidson

Open Access

https://doi.org/10.1007/s00707-016-1754-7

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Abstract

Direct numerical simulations of a turbulent channel flow at low Reynolds number (Re_{tau } = 180, based on the driving pressure gradient and channel half width) are performed. Some results are also presented for Re_{tau } = 400. In this work we apply an idealized spanwise Lorentz force near the lower wall of the channel and compare the results for the applied force and no-force cases in both the upper half and the lower half of the channel. We have studied two-point correlations to explain the effect of the Lorentz force on streamwise vortices and streaky structures. Despite the observation of clear stabilization of the streaky structures in the vicinity of the wall, the existence of the streamwise vortices is explained by the well-known turbulence regeneration cycle, which improves the understanding of streaky and streamwise vortex structure formation on turbulence generation. Spanwise oscillating Lorentz force effects on the Rankine vortex structures are investigated. Our results lead us to establish an explanation on the effect of sweep and ejection events on the mean vortex structures in the flow field. A mean vortex structure is defined by the time-averaged location of the local minimum and maximum of the streamwise r.m.s. vorticity. We also depict turbulence production rates for both cases and compared the lower and upper half of the channel.

Highlights

Flow control has become increasingly important both in terms of environmental health and economic benefits.1 One of the methods used to study the flow control in wall-bounded flows is to apply an oscillating Lorentz force
The DNS simulations are performed for a turbulent channel flow (Reynolds number of 180 and 400) in which Lorentz force excitation is applied along the spanwise direction in order to investigate the potential for drag reduction
For the no-force case we observe a larger intercept for the log law for Reτ = 180, compared to the Reτ = 400 case (Fig. 3) and the urms profile exhibits a peak at y+ = 14 (Fig. 4), which is in agreement with Moser et al [28]

Summary

Introduction

Flow control has become increasingly important both in terms of environmental health and economic benefits. One of the methods used to study the flow control in wall-bounded flows is to apply an oscillating Lorentz force. Huang et al [18] applied a spanwise oscillating Lorentz force at the lower wall of a turbulent channel flow and achieved a drag reduction They noted that a negative spanwise vorticity is generated which makes the streaks tilt and oscillate. Kim [23] showed that the near-wall streamwise vortices are the single most important turbulent structure when studying drag reduction This is supported by the observation that streamwise vortices have been found to be responsible for both ejection and sweep events of the bursting process [34]. The DNS simulations are performed for a turbulent channel flow (Reynolds number of 180 and 400) in which Lorentz force excitation is applied along the spanwise direction in order to investigate the potential for drag reduction. The results are presented and discussed, and some concluding remarks are given in the final section

Governing equations

Direct numerical simulations

Results and discussion

Conclusion